This invention relates generally to container lids, and to methods for forming them and applying them to containers.
Virtually all plastic lids, whether used to cover plastic containers such as margarine tubs or as "overlids" for resealing opened metal containers such as coffee cans, are applied to containers with automatic applicator machines that utilize a rotating rod mechanism to automatically feed the lids along a path. This mechanism consists of a pair of substantially horizontally oriented and substantially parallel rotating rods between which the lids sit on edge. To aid the advancement of the lids with gravity, the rods may be tilted or angled when viewed in side elevation. Also, the rods may deviate from true parallel by diverging or spreading apart in the direction of lid travel. This rotating rod mechanism is also used to transport lids in manufacturing and printing operations.
Previous lids have a preferential tendency to move along the rods in the direction of the bottom of the lid. This is due to the slightly tapered edge or skirt surrounding the perimeter of the lid. Although the mold in which the lid is formed has straight side walls, the skirt "toes in" or becomes narrower the farther it is from the top of the lid, due to the tendency of the skirt to shrink upon cooling to a greater degree at the bottom of the skirt than at the top. The bottom end of the skirt has a large bead protruding internally, which snaps into engagement over a rim on the top of the container to be covered. The difference in shrinkage rates along the skirt results from the variation in thickness of the skirt due to the bead. The lids advance along the rods in the narrowing direction of the taper, regardless of the rotational direction of the rods.
For specialized applications, lids have been produced with a taper which will cause them to walk along the rods in the opposite direction, i.e., towards the top of the lid. This was accomplished by including an external ridge around the bottom of the skirt, which compensated for the taper due to shrinkage, and created a taper in the opposite direction. Thus, the outside diameter of the lid would vary from largest at the bottom of the skirt to smallest at the top of the skirt. These ridged lids are useful only in relatively few circumstances where the machinery is adapted for the lid to advance top-side-forward.
In contrast, shrinkage tapered lids have become widely accepted within the industry and are necessary for the operation of lid applicator machines designed on the assumption that the lids will feed along the rods bottom-side-forward. However, as the thicknesses and lengths of the skirts have become smaller to save material, it has become increasingly difficult to get a consistent taper or toe-in using the shrinkage technique. As a result, shrinkage tapered lids do not feed uniformly on the rotating rods. Some lids feed more slowly than others, creating gaps in the advancing stack of lids on the rods. This decreases the pressure on the lids at the end of the rods which feeds the lids into the applicator. This translates into a lid application rate of at most, 150 containers per minute, despite some applicator's designed capacity of 400 containers per minute.
This low rate of lid application requires either investment in more applicator machines or results in congestion or a backlog at the lid application stage of the capping process. Additionally, inconsistencies in the movement of the lid supply on the rods requires manual intervention in an otherwise automatic process, and may result in uncapped containers being automatically processed in the subsequent packaging operations.
Previous attempts at fabricating a lid with a consistently tapered skirt have been unsuccessful. Typically, plastic lids are injection molded in a two-piece mold comprised of a mold cavity which forms the outer, top surface of the lid, and a removable mold insert which forms the inner, bottom surface of the lid. If the skirt were to be significantly tapered, or narrowed towards the bottom, by forming a mold cavity and insert with inwardly sloped or "reversely-tapered" side walls, it would be impossible to remove the mold insert or the lid from the mold cavity since the mold insert and lid would effectively be dove-tailed within the mold cavity.
One solution is to use a split or two-piece mold cavity, however this increases the cost of the mold itself and slows the molding process to a degree which makes the lid prohibitively expensive to produce. Even if the lid could be removed from a one-piece mold cavity with walls angled for a reverse taper, there is a limit to the amount of taper which can be created, since as the inside diameter of the skirt narrows due to the taper, it becomes more difficult to fit the lid over the container.
Thus, a need exists for an inexpensively manufactured container lid which is able to consistently move bottom-side-forward along a pair of spinning rods.